This invention relates to an apparatus for inflating pneumatic tires of different sizes by first inflating a tire to a predetermined overpressure and then bleeding it to the desired final tire pressure, the apparatus being of the type comprising an inlet and an outlet for pressurized air and pressure regulating means mounted therebetween, the outlet being connectable with the tire, the pressure regulating means comprising a pressure gauge such as a contact manometer and switch valve means, which are controlled by a timer so as to alternatingly, during the inflation, connect the outlet with the inlet and the pressure gauge, respectively, to thereby produce alternating inflation periods and measuring periods, until the attainment of the predetermined overpressure is detected by the pressure gauge, which is operatively connected with control means for causing the switch valve means to alternatingly connect the outlet with the pressure gauge and an exhaust opening, respectively, in response to the attainment to produce alternating bleeding periods and measuring periods until the desired tire pressure is detected by the pressure gauge.
Especially for large size tires for trucks etc. it is desirable to effect the inflation in a well controlled automatic manner, and it is an official security requirement that the tires should be inflated to an overpressure and then bleeded to the final pressure. In practice it is of course important that the inflation is carried out as rapidly as possible, but the rapidity is counteracted by the fact that the actual tire pressure is not practically measurable constantly during the inflation, because a pressure gauge as simply connected with the inflation pipe or hose will not necessarily detect the tire pressure, but rather the inflation pressure in front of the narrow air inlet of the tire. A correct measurement of the tire pressure will require disconnection between the tire and the applied source of pressurized air, whereafter the tire pressure is measurable separately.
The said known type of apparatus is designed to operate according to this principle. The length of the measuring periods is chosen so as to ensure that the air pressure in the tire gets time to be propagated all the way to the pressure gauge, i.e. for securing a correct measuring of the pressure. The length of the inflation periods is chosen as a compromise between vey long periods, which would produce a resulting high inflation rate, because the inflation would be interrupted by the measuring periods few times only, and rather short periods, which would of course result in a low rate of inflation, but, however, also in a high accuracy with respect to the attainment of the desired tire pressure. According to the said compromise, a reasonable and necessary accuracy is achieved for an acceptable total rate of inflation, or rather an inflation rate having to be accepted by the users.
It is the purpose of the invention to provide an apparatus of the type referred to, which is controllable for optimizing the said compromise in an easy manner. The invention is based on the recognition that for a tire of slightly increased volume a given inflation accuracy will be obtainable with the use of a slightly increased average rate of inflation, and that such an increase or adjustment may be effected by causing the said timer to differentiate the length of the inflation periods in accordance with the size or volume of the individual tire to be inflated.
Accordingly, the apparatus of the invention is characterized in having selector means for the reading in of the volume of the tire to be inflated, these selector means being operatively connected with the timer in such a manner that the timer causes the length of the inflation periods to be variably adjusted substantially in accordance with the read in volume of the tire.
It will not be necessary for the selector means to cause any change of the length of the measuring periods, because the optimal length of these periods is almost an apparatus constant referring to the tire pressure propagating to the pressure gauge.
Tires are not normally provided with indication of their volume, but with indication as to their diameter and width, and the volume can be calculated on that basis. According to the invention, therefore, it is possible to combine the selector means with a volume calculator unit and then rely on a reading in of the tire diameter and width only, whereby the selector means may present themselves as two selector knobs cooperating with scales for tire diameter and width, respectively. Very little skill will then be required for the correct setting of the selector means.
The major part of the total inflation time refers to the inflation up to the predetermined overpressure, while the following bleeding is more rapidly accomplished. It could be acceptable, therefore, that the bleeding be effected by way of short duration bleeding periods between the measuring periods, so as to meet the accuracy requirements of small size tires with respect to the final tire pressure, while then larger sized tires would be bleedable with an even higher accuracy. However, when such increased accuracy is not required, it is more important to achieve a speeding up of the process even in this final phase thereof, and according to the invention, therefore, it is highly advantageous to let the selector means control not only the inflation periods, but also the corresponding bleeding periods.